To group content together on the splash pages you can add a Group.xml file to your product splash page article. This will use the sort_order of the books and the content of the Groups.xml file to group books.
The format of the Gropups.xml file is a variablelist, with a varlistentry for each group you wish to have on the site.
e.g. to group all the default (sort_order 50) books together with a group name and a description:
<para>Books for Users and other people not developing stuff</para>
Note: the role="50" attribute in the varlistentry is the magic that ties the group to the books.
The web logic simply inserts the varlistentry output in to the splash pages on the website before the first book with that sort_order for a given product version. There must be at least one book with a matching sort_order for the group to be included.
The para in the list item must exists, however if it is empty '<para></para>' then it will be excluded from the output. This will mean only a group name is displayed on the web site.